Bugfix: mark edited proof as obsolete if the answer of prover is unknown.
Bugfix: mark edited proof as obsolete if the answer of prover is unknown.

If the answer of prover was unknown and reason of the answer was not None
and the proof was edited, it was not marked as obsolete. This commit
fixes this problem.

Contributed by Stefan Berghofer.

ssaavedra/why3,ssaavedra/why3,ssaavedra/why3